mm'''-' 


mr^'M- 


),0  «■■-','''" 


'r„.;(;i'/? 


;?^■''■''t:!:^•■■^''■'V  "  :i:/\li^'','';-l/:'^^ 


trmm 


I 


S® 


.?:«?;•' 


:^r'»- 


l| 


^t:^; 


Stages  in  the  Development 


OF 


SlUM   CICUTAEFOLIUM 


BY 


GEORGE  HARRISON   SHULL 


WASHINGTON,  D.  C. : 

Published  by  the  Carnegie  Institution  of  Washington 

May,  1905 


•  f  4  3 


Stages  in  the  Development 


OF 


SlUM    CICUTAEFOLIUM 


BY 


GEORGE  HARRISON   SHULL 


WASHINGTON,  D.  C. : 

Published  by  the  Carnegie  Institution  of  Washington 
May,  1905 


Carnegie  Institution  of  Washington,  Publication  No.  30. 


Papers  of  Station  for  Experimental  Evolution 
AT  Cold  Spring  Harbor,  New  York.    no.  3. 


PRESS    OP 

HENRY   E.  WILKENS   PRINTING  CO. 

WASHINOTON.   O.   C. 


PLATE  I. 


Sium  ciciilue/oliuiii  (jiiit-l.  A  jar  <il  secdIiiiKS,  piioto^^raplied  at  llie  University 
of  Chicago,  Octolier  lo,  1903,  by  Ur.  W.  J.  G.  Land  ;  spring-growtii  of  an  old 
plant,  piiotograpried  at  Lockport,  ill.,  May  16,  1903,  by  W.  E.  Praeger  ;  and  a 
flowering-stem  piiotograplied  at  South  Cliicago,  September  22,  1903. 


Stages  in  the  Development  of  Sium 
cicutaefolium. 


By  George  Harrison  Shull. 


The  juvenile  forms  of  leaves  have  been  believed  to  be  so  related  to  the 
evolutionary  history  of  a  plant  as  to  indicate  the  form  of  leaf  possessed 
by  its  ancestors  and  to  furnish  satisfactory  evidence  of  the  closeness  of 
relationship  between  allied  species.  It  is  impossible,  in  most  cases,  to 
determine  the  ancestors  of  any  species,  and  it  is  likewise  impossible, 
therefore,  to  demonstrate  a  close  parallel  between  ontogeny  and  phy- 
logeny.  As  there  is  in  both  processes  the  development  from  some  simple 
condition  to  one  and  the  same  complex  condition,  namely,  the  climax 
type  of  leaf  of  the  present  adult  plant,  we  can  scarcely  escape  the  belief 
^  that  such  a  parallel  does  exist  in  many  cases ;  but  how  safely  or  in  how 
""  minute  detail  we  may  reason  from  ontogeny  to  phylogeny  may  well  be 
^]      considered  an  open  question. 

en         The  hypothesis  of  von  Baer  ( 1828)  has  proved  a  very  suggestive  one, 
^      and,  like  most  suggestive  hypotheses,  has  been  given  a  much  wider  ap- 
plication than  its  author  would  have  been  willing  to  sanction.    Yon  Baer 
did  not  assume  that  the  adult  characters  of  the  ancestors  occur  as  larval 
or  juvenile  characters  in  the  descendants,  but  that  the  same  larval  stages 
o        occur  in  both,  a  given  stage  appearing  earlier  in  the  descendant  than  in 
^  ©  the  ancestor.     The  idea  that  larval  and  juvenile  characters  agree  with 
fZ  C3  ancestral  adult  characters  was  an  old  conception  which  was  rehabilitated 
2.  S  by  Louis  Agassiz  (1848-1849)  and  became  crystallized  in  the  epigram 

CO  '  "^ 

i^j  ^  of  Haeckel  (1866),  which  is  now  universally  known  as  the  "Law  of 

ij-i  ^  biogensis  " — that  "  ontogeny  repeats  phylogeny."* 

^  '^      Great  stress  was  laid  upon  this  hypothesis  by  Hyatt,  Cope,  and  others 

^  who  have  used  it  as  a  most  important  principle  in  disentangling  difficult 
phylogenetic  problems.  More  recently  Jackson  (1899)  has  called  atten- 
tion  to   the    fact   that   in   organisms    having   periodically    interrupted 


CD 


*For  a  [history  of  the  development  of  tlie  idea  of  repetition  see  Hyatt,  A., 
"  Cycle  in  the  life  of  the  indvidual  and  in  the  evolution  of  its  own  group."  The 
law  of  repetition  is  there  called  "  Agassiz's  law  of  palingenesis."  See  also 
Glaser,  O.  C,  "  The  law  of  von  Baer." 

(3) 


'•  !•  "3  (T*  r*  <T  "O 


4  DEVELOPMENT    OF    SIUM    CICUTAEFOLIUM. 

growth,  modified  characters  which  he  beheves  to  be  atavistic  occur  in 
definite  parts  of  the  organism,  particularly  in  the  region  where  resump- 
tion of  growth  takes  place.  These  local  modifications  he  calls  "  local- 
ized stages  "  and  says  that  the  equivalents  of  these  stages  "  are  to  be 
sought  in  the  adults  of  ancestral  groups."  In  three  recent  papers  in  the* 
American  Naturalist,  Cushman  (1902,  1903,  1904)  gives  the  result  of 
studies  similar  to  those  of  Jackson,  and,  besides  presenting  evidence  that 
at  the  resumption  of  growth  in  perennial  herbs  in  the  spring  these  ata- 
vistic modifications  occur,  lays  stress  upon  senescent  stages  as  showing 
even  more  primitive  conditions  than  are  to'be  found  in  the  seedling.  It 
may  be  said,  in  passing,  that  this  view  of  the  significance  of  senescent 
stages  does  not  fully  accord  with  that  of  Hyatt  (1890,  pp.  78,  79;  1897, 
p.  221),  who  looked  upon  senescent  stages  as  indicative  of  the  course 
any  species  in  question  will  pursue  in  the  process  of  degenerative  evo- 
lution. In  Hyatt's  view  the  senescent  stages  have  a  prophetic  instead 
of  an  historic  significance,  though  he  recognized,  of  course,  that  there 
are  many  resemblances  between  the  senescent  and  the  juvenile  series. 

If  the  propositions  of  Hyatt  and  Cope,  of  Jackson,  and  of  Cushman 
are  all  true,  there  should  be  at  least  three  regions  in  any  perennial  plant 
which  will  agree  in  presenting  ancestral  characters — the  juvenile  leaves 
following  the  cotyledons,  the  earliest  formed  parts  at  each  renewal  of 
vegetative  activity,  and  the  senescent  stages  approaching  and  accom- 
panying the  inflorescence.  In  numerous  cases  there  is  a  general  agree- 
ment in  the  forms  passed  through  at  these  three  regions,  and  no  incon- 
sistency arises  when  they  are  all  looked  upon  as  atavistic,  though  the 
mere  fact  of  agreement  in  the  several  regions  can  not  be  taken  as  in 
itself  convincing  evidence  that  these  characters  agree  with  the  adult 
characters  of  some  ancestral  group.  On  the  other  hand,  certain  plants 
do  not  show  the  same  modification  of  leaf-form  in  the  inflorescence  that 
is  found  in  the  "  nepionic  "  leaves  of  the  seedling,  and  it  becomes  at  once 
evident  that  no  reliance  can  be  placed  upon  the  forms  of  leaf  occurring 
in  any  of  these  regions  as  having  more  than  the  most  general  signifi- 
cance as  indications  of  ancestral  characters. 

A  plant  which  most  strikingly  illustrates  this  fact  is  the  hemlock 
water-parsnip  {Sium  cicutaefolium  Gmel.),  which  presents  a  great 
range  of  leaf-form  and  passes  rather  rapidly,  sometimes  suddenly,  from 
one  form  to  another  without  repetition,  so  that  each  of  the  regions  sup- 
posed to  tell  of  ancestral  conditions  tells  a  different  story.  This  fact  is 
illustrated  in  a  general  way  by  Plate  I. 

The  seedling  of  Sium  cicutaefolium  is  so  different  from  the  adult 
plant  that,  except  when  the  two  are  associated  together,  its  identity 


DEVELOPMENT    OF    SIUM    CICUTAEFOLIUM.  5 

would  scarcely  be  suspected.  I  am  not  aware  that  this  seedling  has  ever 
been  described,  and  the  striking-  variations  in  the  form  of  the  juvenile 
leaves  make  description  difficult. 

The  cotyledons  are  quite  similar  to  those  of  many  other  of  the  Um- 
belliferae,  being  narrowly  oblong  or  elliptic  oblong,  rather  acutely 
rounded  at  the  apex,  and  tapering  gradually  into  a  petiole  at  the  base. 
The  petiole  and  blade  together  are  1.5  to  2  cm.  long  and  the  blade  is 
2.5  to  4  mm.  wide. 

The  early  "  nepionic  "  leaves  are  extremely  variable,  but  it  is  rare 
that  they  do  not  consist  of  a  single  blade  constructed  on  a  somewhat 
palmate  plan.  The  blade  is  usually  of  rounded  form  and  quite  variously 
notched,  cut,  or  lobed.  In  order  to  facilitate  a  systematic  investigation 
of  the  variation  in  these  leaves  they  were  divided  into  several  more  or 
less  artificial  groups.  The  first  nepionic  leaf  in  more  than  120  unse- 
lected  individuals*  presented  forms  which  were  placed  in  six  of  these 
categories,  as  follows : 

(a)  About  10.5  per  cent  were  divided  palmately  into  five  nearly  equal 
lobes,  one  or  two  of  which  were  sometimes  slightly  notched. 

(b)  Twenty  per  cent  were  3-lobed,  with  the  middle  lobe  2-notched 
and  the  lateral  lobes  each  bifid. 

(c)  The  leaves  in  the  third  group  were  likewise  3-lobed,  but  the  lobes 
were  quite  variously  notched.  Over  28  per  cent  of  the  seedlings  had 
the  first  leaf  of  this  description. 

It  will  be  noted  that  the  form  of  leaf  described  under  (b)  is  simply  a 
special  case  of  (c),  and  was  separated  from  it  because  this  was  the  only 
form  of  trilobed  leaf  which  could  be  so  definitely  characterized,  and 
because  this  simple  definite  type  of  trilobation  is  almost  wholly  limited 
to  the  first  nepionic  leaf. 

(d)  Over  15  per  cent  showed  the  tendency  to  trilobation  by  a  single 
cleft  on  one  side,  unmatched  by  a  cleft  on  the  opposite  side.  These 
will  be  spoken  of  as  "  half  3-lobed  leaves."  They  pass  gradually  into 
a  form  having  one  lateral  leaflet,  and  these  two  conditions  were  kept 
together  in  a  single  category. 

(e)  Nearly  25  per  cent  had  a  generally  rounded,  ovate  or  cordate 
form,  merely  dentate,  or  irregularly  cleft  in  a  manner  which  did  not 
suggest  trilobation. 


*The  seedlings  were  taken  up  on  two  small  clumps  of  earth  from  their  native 
(habitat  near  Robej'-,  south  of  Chicago,  Illinois.  The  individual  seedlings  were 
carefully  separated  and  all  were  transplanted  about  3  cm.  apart  in  a  regular  pat- 
tern in  glass  dishes.  The  point  of  departure  for  each  dish  was  marked  and  a 
number  assigned  to  each  seedling,  so  that  drawings  made  from  time  to  time  could 
be  labeled  in  a  manner  to  allow  .the  progress  of  each  individual  to  be  followed. 


6  DEVELOPMENT    OF    SIUM    CICUTAEFOLIUM. 

(f)  Less  than  2  per  cent  had  one  pair  of  lateral  leaflets,  and  may 
perhaps  be  looked  upon  as  an  extreme  form  of  the  3-lobed  condition  of 
category  (c),  but  (c)  is  essentially  palmate,  while  (/)  is  essentially 
pinnate. 

A  glance  at  Plate  II  will  show,  better  than  description,  the  variability 
of  the  first  nepionic  leaf,  and  it  will  also  show  how  truly  gratuitous  is 
the  division  into  the  categories  just  described.  The  leaves  were  chosen 
at  random  from  the  material  at  hand,  but  when  more  leaves  were  chosen 
than  could  find  place  in  the  plate  the  less  dubious  forms  w^ere  discarded 
in  order  to  show  the  real  difficulties  of  such  a  classification  and  to  give 
a  clue  to  the  personal  equation  of  the  writer  in  distinguishing  the  several 
categories.  The  letters  A,  B,  C,  etc.,  illustrate  the  categories  described 
above  under  (a),  (b),  (c),  etc. 

The  first  two  classes,  (a)  and  (b),  are  characteristic  of  the  first  nepi- 
onic leaf.  With  the  exception  of  these  two  forms,  all  the  categories  de- 
scribed above  are  represented  in  each  succeeding  leaf  up  to  the  sixth, 
and  perhaps  occasionally  in  the  seventh,  and  in  these  later  leaves  the 
various  types  are  much  more  definitely  distinguishable,  so  that  there  is 
rarely  any  question  as  to  where  any  leaf  should  be  classed.  With  rare 
exceptions  every  leaf  after  the  sixth  is  pinnate  and  shows  a  marked 
contrast  to  the  great  variability  of  the  first  leaf,  so  that  in  the  eighth  leaf 
a  very  large  majority  have  three  pairs  of  lateral  leaflets  which  closely 
resemble  the  typical  stem-leaves  of  the  species,  except  in  texture  and  in 
the  more  ovate  form  of  the  terminal  leaflet. 

In  the  second  nepionic  leaf  (Plate  111,2)  trilobation  was  found  in 
only  a  little  over  15  per  cent  of  the  seedlings,  or  about  one-third  as  fre- 
quently as  in  the  first  nepionic  leaf,  but  the  half  3-lobed  form  had  in- 
creased from  15  per  cent  to  18  per  cent,  the  unlobed  or  irregularly  lobed 
form  had  increased  from  25  per  cent  to  over  61  per  cent,  and  the  form 
with  one  pair  of  lateral  leaflets  from  1.6  per  cent  to  about  5  per  cent. 
Just  as  the  3-lobed  form  was  the  modal  condition  of  the  first  nepionic 
leaf,  the  unlobed  form  was  the  modal  condition  of  the  second. 

The  third  nepionic  leaf  (Plate  III,  3)  showed  a  partial  return  to  the 
3-lobed  condition,  and  nearly  24  per  cent  of  the  specimens  were  so  char- 
acterized. The  half  3-lobed  form  occurred  in  over  22  per  cent,  the  un- 
lobed or  irregularly  lobed  form  in  39  per  cent,  and  the  number  having  a 
pair  of  lateral  leaflets  had  increased  from  5  per  cent  in  the  second  leaf 
to  over  15  per  cent  in  the  third.  Here,  although  there  was  an  increase 
in  each  of  three  categories  at  the  expense  of  the  unlobed  form,  the  latter 
was  still  the  modal  form  of  leaf. 

The  fourth  leaf  (Plate  III,  4)  was  3-lobed  in  24  per  cent,  half  3-lobed 
in  15  per  cent,  unlobed  or  irregular  in  nearly  23  per  cent,  and  pinnate 


PLATE  II. 


B 


s? 
<? 


'^ 


^ 


^  <ii 


O'^o  .55* 


Variation  in  the  first  "nepionic"  leaf  of  Stum  cicutaefolium.  Eight-ninths 
natural  size.  All  drawn  from  nature  with  an  Abbe  camera.  A,  B,  C,  etc.,  cor- 
respond to  (a),  (<J),  {c),  etc.,  in  the  text. 


PLATE  III. 


Variation  in  successive  seedling-leaves  of  Slum  cicutae/olium,  from  the  second  to 
the  seventh.  Eight-ninths  natural  size.  Drawn  from  nature  with  an  Abbe 
camera.    Numbers  represent  the  position  of  each  group  of  leaves  in  the  seedling. 


DEVELOPMENT    OF    SIUM    CICUTAEFOLIUM. 


with  one  pair  of  lateral  leaflets  in  30  per  cent,  this  simplest  type  of  pin- 
nation  now  becoming  the  modal  form  of  leaf. 

In  the  fifth  leaf  (Plate  III,  5)  the  percentages  of  all  the  types  of  single 
blades  had  considerably  decreased  and  the  pinnate  had  increased,  so  that 
now  only  4.3  per  cent  were  3-lobed  and  the  same  number  half  3-lobed, 
17  per  cent  were  unlobed  or  irregular,  and  more  than  71  per  cent  had 
one  pair  of  lateral  leaflets,  while  a  little  more  than  2  per  cent  had  two 
pairs  of  leaflets. 

The  sixth  leaf  was  3-lobed  in  only  3.5  per  cent  of  the  specimens  ex- 
amined and  the  same  number  were  unlobed  or  irregular.  About  7  per 
cent  were  half  3-lobed,  nearly  45  per  cent  had  one  pair  of  leaflets,  and 
over  41  per  cent  had  two  pairs  of  leaflets. 

In  the  seventh  leaf  none  were  observed  which  had  not  at  least  one 
pair  of  leaflets,  though  it  is  not  improbable  that  single  blades  may  occur 
occasionally  in  the  seventh  leaf.  The  specimens  in  hand  showed  27.5 
per  cent  with  one  pair  of  leaflets,  over  51  per  cent  with  two  pairs  of  leaf- 
lets, and  21.6  per  cent  with  three  pairs  of  leaflets,  so  that  in  the  seventh 
nepionic  leaf  the  form  with  two  pairs  of  lateral  leaflets  is  the  modal 
form. 

The  eighth  leaf  presents  the  same  three  forms,  but  only  a  little  over 
9  per  cent  had  a  single  pair  of  lateral  leaflets,  18  per  cent  had  two  pairs, 
and  nearly  y^  per  cent  had  three  pairs. 

For  more  ready  comparison  these  percentages  are  brought  together 
in  the  following  table.  The  modal  form  (empirical)  in  each  leaf  is 
given  in  full-faced  type.  The  variation  of  leaf-form  occurring  in  each 
succeeding  nepionic  leaf  from  the  second  to  the  seventh  is  shown  in 
Plate  III.  The  left  and  right  sides  of  the  figure  show  the  extreme 
range  of  form  found  in  each  leaf.  The  index  figures  at  the  left-hand 
side  of  the  plate  indicate  the  position  of  the  leaves  with  reference  to  the 
cotyledons. 


Form  of  leaf. 

Position  of  leaf. 

I 

2 

3 

4 

5 

6 

7 

8 

(a)   Five-lobed 

(6)  Three-lobed 

19.5 
19.4 
28.2 

15-3 

25.0 

1.6 

1.6 
32.6 

14.5 
22.6 
38.7 

(c)   Three-lobed 

15.7 

18.2 

61.2 

5-0 

23-7 
22.0 
39.0 

15.3 

4.3 
4.3 
17.0 

72.3 

2.1 

34 

6.9 

34 

44.8 

414 

(d)  Half  3-lobed 

(<?)   Unlobed  or  irregular 
(0   One  pair  of  leaflets. . 
(g)  Two  pairs  of  leaflets. 
(/»)  Three  pairs  of  leaflets 

27.0 
51.4 

21.6 

91 
18.2 
72.7 

! i 

8  DEVELOPMENT    OF    SIUIVI    CICUTAEFOLIUM. 

If  the  several  categories  into  which  these  leaves  have  been  divided 
are  examined  critically,  the  following  facts  will  be  noted : 

1.  They  are  not  all  of  equal  value.  The  second  category  (&)  is  only 
a  special  case  of  the  third  (c).  The  fourth  (d)  is  transitional  between 
(c)   and   (e).     The  fifth   (e)   is  Composite,  but  not  readily  divisible. 

2.  Some  of  the  distinctions  are  qualitative,  i.  e.,  complexly  quantita- 
tive, as  exemplified  by  (b),  (c),  (e),  etc.;  others  are  simply  quantita- 
tive, as,  for  example,  (f),  (g),  and  (h). 

3.  They  are  not  arranged  in  a  logical  linear  series  and  are  incapable 
of  being  so  arranged.  They  have  been  arranged  in  the  order  in  which 
they  become  dominant  during  development  and  without  reference  to 
their  logical  relations.  The  undivided  or  irregularly  cleft  blade  so 
characteristic  of  the  second  nepionic  leaf  is  the  logical  termination  of 
a  line  of  development  from  the  3-parted  leaf  through  the  half  3-parted, 
but  the  pinnate  leaf  with  one  pair  of  lateral  leaflets,  instead  of  joining 
directly  to  this  undivided  form  which  it  follows,  also  comes  logically 
from  the  3-parted  type.  If  the  progress  of  the  phylogeny  of  the  species 
is  correctly  indicated  by  this  ontogenetic  series  it  would  appear  that 
after  a  gradual  departure  from  the  ancestral  tripartite  leaf  to  the  undi- 
vided leaf  there  had  been  a  sudden  jump  or  mutation  to  the  opposite 
extreme  in  which  the  clefts  of  the  tripartite  leaf  became  so  exaggerated 
as  to  leave  the  lateral  lobes  separated  from  the  terminal  by  a  space,  thus 
forming  a  pinnate  leaf.  It  can  not  rightly  be  either  affirmed  or  denied 
that  such  mutation  took  place  in  the  history  of  the  species,  but  it  appears 
to  me  that  any  attempt  to  interpret  the  details  of  ontogenetic  develop- 
ment as  corresponding  so  definitely  to  phylogenetic  development  would 
be  pressing  the  analogy  to  a  wholly  unwarranted  degree. 

In  fig.  I  the  variation  in  the  several  nepionic  leaves  from  the  first  to 
the  eighth  is  represented  graphically  in  the  form  of  curves,  the  several 
categories  being  treated  as  if  of  equal  value.  Although,  as  we  have  just 
seen,  this  is  not  strictly  true,  the  figure  still  allows  the  derivation  of  at 
least  one  interesting  conclusion — there  is  a  progressive  lessening  of  the 
variability  from  the  first  leaf  onward.  The  modal  class  in  the  first  nepi- 
onic leaf  contained  only  28.2  per  cent  of  the  individuals,  or,  if  we  should 
include  in  this  class  the  special  form  of  3-parted  leaf  which  has  been 
separated  as  a  distinct  category,  it  would  still  contain  only  47.6  per  cent 
of  the  individuals,  while  the  modal  class  in  the  eighth  leaf  included 
nearly  y;^  per  cent  of  the  specimens,  and  the  variability  within  the  class 
was  very  much  less  in  the  latter  than  in  the  former. 

There  are  several  ways  in  which  we  may  account  for  the  great  varia- 
bility in  the  earliest  leaf  and  the  decreasing  variation  with  subsequent 


DEVELOPMENT    OF    SIUM    CICUTAEFOLIUM.  9 

development.  If  these  juvenile  leaves  do  correspond  with  adult  condi- 
tions of  the  past  we  might  suppose  that  at  the  time  when  the  species  was 
characterized  by  a  tripartite  leaf  it  was  much  more  variable  than  it  be- 
came later,  and  that  the  process  of  evolution  acted  to  give  it  gradually  a 
more  and  more  fixed  character.  Or,  on  the  other  hand,  it  might  be  sup- 
posed that  the  species  has  been  at  all  times  about  equally  variable,  but 
that  by  the  "  law  of  acceleration  "  the  first  nepionic  leaf  represents  a 
much  longer  period  in  the  history  of  the  species  than  does  any  subse- 
quent leaf.     Again,  it  may  be  that  these  leaf-forms  are  not  atavistic,  and 


Fig.  1  .—Graphic  representation  of  the  variation  In  the  several  "  nepionic  " 
leaves  of  Sium  cicutae/olium,  from  the  first  to  the  eighth,  inclusive. 
Ordlnates  given  in  per  cents. 


the  wider  range  of  variation  may  merely  represent  a  greater  sensitive- 
ness to  minute  variations  of  the  internal  and  external  environment.  As 
the  plant  becomes  more  and  more  firmly  established  it  becomes  morpho- 
logically more  s6lf-determinative.  In  other  words,  the  greater  variabil- 
ity and  the  gradual  decrease  in  variability  may  be  purely  physiological 
facts  unrelated  in  any  direct  way  to  the  phylogenetic  history  of  the 
species.  It  is  possible  indeed  that  each  of  these  propositions  mat  rep- 
resent a  partial  truth,  as  they  are  not  mutually  exclusive.  The  species 
may  have  been  more  variable  in  its  adult  leaf-form,  the  law  of  accelera- 
tion may  give  rise  to  a  greater  range  of  atavistic  variation  in  the  earlier 


10  DEVELOPMENT    OF    SIUM    CICUTAEFOLIUM. 

leaves,  and  physiological  causes  may  determine  which  atavistic  form 
shall  appear  and  may  in  addition  give  rise  to  forms  or  details  of  form 
which  are  in  no  sense  atavistic. 

A  point  in  favor  of  the  interpretation  of  the  tripartite  leaf  as  being  at 
least  in  a  general  way  atavistic  may  be  found  in  the  fact  that  ternate 
division  of  leaves  is  notably  prevalent  among  the  Umbelliferae ;  but  it 
should  not  be  forgotten  that  a  ternate  condition  is  a  necessary  logical 
transitional  stage  between  any  "  simple  "  leaf  and  a  pinnate  leaf,  and  its 
significance  must  rest  solely  upon  this  logical  relation.  To  show  how- 
gratuitous  is  any  attempt  to  draw  from  the  juvenile  leaves  conclusions 
regarding  the  leaf- form  in  ancestral  groups  it  is  only  necessary  to  inves- 
tigate the  conditions  found  in  the  few  instances  in  which  the  ancestry 
of  a  species  is  definitely  known.  Through  the  kindness  of  Dr.  D.  T. 
MacDougal,  of  the  New  York  Botanical  Garden,  I  am  enabled  to  pre- 
sent photographs  of  the  seedlings  and  adult  rosettes  of  Onagra  laniarck- 
iana  and  O.  rubrinervis,  the  former  species  being  the  parent  of  tlie 
latter.  They  represent  the  closest  relationship  possible  between  two 
species,  and  yet  those  who  are  experienced  in  their  culture  separate 
them  with  unfailing  accuracy  in  the  earliest  juvenile  stages,  A  com- 
parison of  the  seedlings  and  the  adults  in  Plates  IV  to  VI  makes  evident 
the  fact  that  the  seedling  of  Onagra  rubrinervis  resembles  much  more 
closely  the  adult  condition  of  the  same  species  than  either  the  seedling 
or  the  adult  condition  of  its  parent  species,  Onagra  laviarckiana. 

Turning  now  to  other  regions  which  have  been  supposed  to  present 
ancestral  types,  particularly  to  the  two  regions  most  strongly  exploited 
by  Cushman  (1902,  1903,  1904)  in  his  recent  papers  in  the  American 
Naturalist,  we  find  conditions  which  are  totally  different  from  those  we 
have  just  considered  in  the  juvenile  stages.  The  first  of  these  regions  is 
that  at  which  a  perennial  herb  resumes  growth  in  the  spring  after  hiber- 
nation. At  the  close  of  the  first  season  from  seed  the  leaflets  of  Sium 
cicutaefolimn  become  more  or  less  dissected  into  narrow  segments,  as 
shown  in  fig.  2.  This  modification  is  very  often  apparently  related  to 
the  more  aquatic  condition  of  the  habitat,  and  is  in  line  with  modifica- 
tions in  many  other  species  whose  dissected  leaves  appear  to  be  corre- 
lated in  some  way  with  aquatic  conditions ;  but  it  can  be  shown  that  this 
change  in  the  form  of  the  leaf  of  Sinm  takes  place,  though  less  per- 
fectly, when  the  plants  are  not  supplied  with  an  unusual  quantity  of 
water.  Plants  taken  up  as  seedlings  in  June  and  grown  under  meso- 
phytic  conditions  in  the  window  of  a  laboratory  at  the  University  of 
Chicago  and  later  in  the  conservatory  of  the  same  showed  dissection  of 
the  leaves  similar  to  that  exhibited  by  plants  that  were  submerged  in 
imitation  of  the  effects  of  autumn  rains. 


PLATE  IV. 


Seedlings  of  Onagra  lamarckiana,  grown  at  the  New  York   Botanical  Garden. 
Photograph  supplied  by  Dr.  D.  T.  MacDougal. 


PLATE  V. 


Seedlinf^s  of  Onagra  rubrinervis  grown  at  the  New  York  Botanical  Garden. 
Photograph  supplied  by  Dr.  D.  T.  MacDougal. 


PLATE  VI 


Large  rosettes  ot  Onagra  lainarckiana  (upper)  and  O.  rubriitervis  (lower),  grown 
at  the  New  York  Botanical  Garden.  Photographs  supplied  bv  Dr.  D.  T. 
MacDougal. 


DEVELOPMENT   OF   SIUM    CICUTAEFOLIUM. 


II 


Burns  (1904)  has  recently  demonstrated  a  similar  independence  from 
aquatic  conditions,  in  the  dissected  leaf  of  Proscrpinaca  palustris, 
though  McCallum  (1902)  had  looked  upon  it  as  a  direct  response  to  the 
dilution  of  the  protoplasm  brought  about  by  the  abundance  of  water  and 
the  stoppage  of  transpiration.  Burns  considers  the  dissected  leaf  of 
Proserpinaca  as  a  juvenile  type,  but  this  is  certainly  not  the  case  in 
Slum  cicutaefoUnni. 

A  more  pronounced,  even  extreme,  dissection  of  the  leaves  character- 
izes the  resumption  of  growth  in  the  spring  (Plates  I  and  VII),  so  that 


Fig.  2.— Autumnal  modlflcation  of  leaves  in  the  seedlings  of 
Siinn  cicutae/olium.  Numbers  represent  the  position  of  each 
leaf  In  the  series,  beginning  with  the  first  neplonic  leaf. 
Three-fourths  natural  size. 


in  SO  far  as  this  region  indicates  the  ancestry  of  the  species  the  ancient 
Slums  should  have  resembled  a  fennel  {Foenicuhim)  or  a  parsley  {Lo- 
matium).  These  dissected  leaves  are  rarely  seen  by  the  systematist  who 
works  only  with  pressed  material,  as  one  may  convince  himself  by  look- 
ing through  the  specimens  of  Shun  in  any  large  hrebarium.  Of  more 
than  100  specimens  of  this  genus  in  the  herbarium  of  the  New  York 
Botanical  Garden  not  one  showed  these  dissected  leaves.  The  change 
from  dissected  leaves  to  the  simply  pinnate  leaves  with  linear,  lanceolate 


12  DEVELOPMENT    OF    SIUM    CICUTAEFOLIUM. 

or  lance-ovate,  sharply  serrated  leaflets  characteristic  of  the  typical  leaf 
of  the  stem,  is  very  sudden  (see  Plates  I  and  VII),  and  would  again 
indicate  a  remarkable  mutation  if  the  course  or  ontogenetic  develop- 
ment agrees  with  the  phylogenetic. 

In  fairness  to  Cushman  it  should  be  said  that  while  this  discussion  of 
the  early  spring  growth  is  based  on  a  literal  interpretation  of  his  papers, 
it  evidently  misrepresents  his  intention.  The  condition  in  Siuin  is  analo- 
gous to  that  in  the  rosette  plants,  in  which  the  writer  had  to  look  care- 
fully beneath  the  rosette  to  find  the  "  primitive "  leaves.  If  he  had 
examined  the  rosettes  at  some  time  during  the  preceding  summer  or 
autumn,  when  they  were  just  beginning  to  develop,  he  would  have 
found  less  difficulty  in  securing  the  leaves  for  which  he  was  looking. 
The  dissected  leaves  in  old  specimens  of  Shun  result  from  the  rejuvena- 
tion of  axillary  buds  at  the  base  of  the  flowering  stem,  and  it  will  be 
seen  later  that  the  first  leaves  of  these  buds  were  of  a  more  juvenile 
type,  but  that  they  belong  to  the  autumn  growth  instead  of  the  period 
of  renewed  vegetative  activity  in  the  spring. 

When  we  examine  the  senescent  stages  approaching  the  inflorescence 
a  series  of  forms  is  found  entirely  diflferent  from  either  of  those  already 
considered,  and  yet  according  to  Cushman  it  is  in  the  region  of  the  in- 
florescence that  we  should  be  able  to  trace  phylogeny  to  its  earliest 
stages.  During  senescence  the  leaves  of  Sium  cicutaefolium  present  a 
marked  contrast  in  every  particular  to  the  juvenile  stages.  The  pro- 
gressive changes  (fig.  3)  present  a  simple  linear  series  instead  of  the 
complex  series  seen  in  the  seedlings,  and  there  is  an  increasing  con- 
stancy of  form  as  senescence  progresses  instead  of  the  increasing  vari- 
ability which  should  be  expected  if  the  senescent  stages  repeat  the  same 
story  as  that  told  by  the  juvenile  stages,  but  in  inverse  order.  Thus  in 
34  adult  plants  examined  with  reference  to  this  point  the  second  leaf 
below  the  principal  inflorescence  (n — 4,  figs.  3  and  6)  had  three  pairs  of 
leaflets  in  10  per  cent,  two  pairs  in  56  per  cent,  and  one  pair  in  34  per 
cent ;  the  leaf  subtending  the  primary  umbel  (n — 3)  had  one  pair  of  leaf- 
lets in  more  than  70  per  cent ;  and  the  peduncle  arising  axillary  to  this 
highest  leaf  of  the  principal  axis  bore  a  bract  (n — 2)  having  a  single 
lance-linear  blade  in  over  80  per  cent.  In  vigorous  plants  peduncles  of 
tertiary  or  still  higher  orders  occur,  and  the  bracts  (n — i  and  n)  on 
these  show  a  regular  reduction  from  the  form  just  described  as  pre- 
dominant on  the  secondary  peduncles  to  small  awl-shaped  structures 
which  apparently  correspond  to  the  base  of  the  petiole  and  leaf-sheath 
of  the  earlier  stem-leaves,  the  blade  having  completely  disappeared. 


DEVELOPMENT    OF    SIUM    CICUTAEFOLIUM. 


13 


Besides  these  differences  in  form  between  the  senescent  and  the  juve- 
nile leaves,  there  is  quite  as  marked  difference  in  the  texture,  venation, 
etc.  The  dissected  leaf  has  already  been  spoken  of  as  having  a  form 
not  rarely  associated  with  aquatic  habitats.  Similarly  the  juvenile  and 
senescent  leaves  have  forms  and  textures  usually  associated  with  meso- 
phytic  and  xerophytic  habitats,  respectively,  the  reduction  of  the  blade, 
firm  texture,  and  strong  cuticularization  of  the  stem-leaves  being  in 
marked  contrast  with  the  broad  blades  and  delicate  texture  of  the  juve- 
nile leaves.     It  will  not  be  surprising  to  find  that  each  of  these  three 


Fig.  3.— The  senescent  series  of  leaf-forms  in  8ium  cicutaefolium. 
Numbered  downward  from  »i,  the  most  reduced  leaf  found  In 
the  Inflorescence;  y  a  terminal  leaflet  showing  Incomplete 
dlfl'erentlatlon  of  a  pair  of  lateral  leaflets.  Two-thirds  natural 
size. 

types  of  leaves  is  associated  with  just  the  conditions  of  environment  to 
which  it  would  appear  to  be  adapted.  The  seedling  develops  in  the 
shade  of  other  plants  during  the  summer,  when  the  substratum  is  less 
wet ;  the  dissected  leaves  occur  in  autumn  and  spring,  when  the  favor- 
ite habitats  of  the  plant  are  apt  to  be  supersaturated  or  flooded  with 
water;  and  the  stem-leaves  are  raised  above  the  substratum,  where 
they  are  more  exposed  to  the  drying  influences  of  wind,  light,  etc.,  and 
at  the  same  time  farther  from  their  water-supply. 

But  too  much  stress  can  easily  be  placed  upon  external  appearance 
as  evidence  of  adaptation,  and  the  dissected  leaves,  which  suggest  by 
their  form  an  adaptation  to  hydrophytic  conditions,  do  not  bear  out  the 
suggestion  when  put  to  the  test.     They  are  unable  to  do  sufficient  pho- 


14 


DEVELOPMENT    OF    SIUM    CICUTAEFOLIUM. 


tosynthetic  work  when  submerged  to  allow  the  plant  to  continue  its 
growth,  and  death  speedily  follows  complete  submergence.  It  might 
be  assumed  that  the  dissection  is  a  product  of  the  environment  instead 
of  an  adaptation  to  it,  and  the  apparent  correlation  of  these  three  stages 
of  Siuni  with  different  environmental  conditions  would  accord  well  with 
the  contention  of  Klebs  (1903)  and  others  that  external  factors  are 
directly  determinative  of  the  form  and  activities  of  plants.  But  the 
plants  which  appear  to  demonstrate  this  proposition  are  quite  excep- 
tional. In  Siuui  cicutaefolinni  this  direct  relation  between  the  form 
and  the  environment  proves  to  be  only  apparent,  and  this  species  seems 
to  agree  with  the  vast  majority  of  plants  in  presenting  a  rather  definite 
cycle  of  development  which  it  passes  through  whenever  the  environ- 
ment supplies  conditions  which  are  favorable  to  growth.  In  such  cases 
the  environment  is  only  indirectly  determinative  of  vegetative  activities. 
It  supplies  the  energy,  but  the  mechanism  of  the  protoplasm  determines 
what  shall  be  the  product,  just  as  the  muscles  of  the  hand  and  arm  used 

in  winding  a  clock  supply 


vegetative  c// 
of  Sporoph_yt^  « 


the  power  which  is  con- 
verted by  the  peculiar 
mechanism  of  the  clock 
into  movement  of  the 
hands,  ticking  of  the  es- 
capement, and  striking  of 
the  hour. 

When  a  cycle  of  vege- 
tative activity  comes  to  a 
close  with  the  senescent 
stages  it  can  be  repeated 
only  as  a  result  of  some 
process  of  rejuvenescence. 
Fertilization  and  seed  pro- 
duction are  to  be  looked 
upon  as  preparatory  to  or 
part  of  the  most  common 
process  of  rejuvenes- 
cence, as  was  pointed  out 
long  ago  by  Alexander 
Braun  (1851),  but  this  is 
not  the  only  manner  in 
which  it  may  be  brought  about.  Whenever  a  bud  grows  immediately 
without  a  resting  period  there  is  no  sign  of  rejuvenescence  and  it  con- 


Gameto- 
phyte 


Spores 

Gametes 

fertilization 

Fio.  4. —Diagram  of  the  life-cycle  of  a  Rpermatophyte, 
showing  the  relation  of  the  process  of  rejuvenes- 
cence as  a  "short-cut"  across  the  cycle.  Upper 
"rejuvenescence"  line  may  represent  the  rejuve- 
nescence of  buda  at  base  of  stem  (flg.  7) ;  the  lower, 
the  proliferations  of  the  flower-buds  (flgs.  8  and  9). 


DEVELOPMENT    OF    SIUM    CICUTAEFOLIUM. 


15 


tinues  the  cycle  on  to  the  close  of  the  senescent  series,  beginning  at  or 
slightly  in  advance  of  the  stage  attained  by  the  axis  at  the  level  from 
which  the  bud  springs.     But  when  it  fails  to  develop  at  once  it  can  not 

resume  activity  except 
through  a  process  of  reju- 
venescence, which  throws 
its  development  back  to 
some  earlier  point  in  the 
cycle,  and  always  at  least 
as  far  down 
the  juvenile 
side  of  the 
cycle  as  the  origin  of  the 
bud  is  down  the  senescent 
side,  a  relation  which  is 
clearly  indicated  by  the 
slant  of  the  "  rejuvenes- 
cence "  lines  in  the  dia- 
gram (fig.  4). 

It  is  this  less  differen- 
tiated, juvenile  condition 
which  usually  occurs  when 
a  resting  bud  is  reawak- 
ened into  active  develop- 
ment which  suggested  to 
Jackson  (1899)  his  con- 
ception of  localized  stages 
having  phylogenetic  sig- 
nificance. 

Some  cases  of  rejuve- 
nescence were  observed  in 
a  specimen  of  Sium  cicii- 
taefolimn  which  had  been 
torn  up  by  the  roots  and 
thrown  down  in  such  a 
way  as  to  submerge  the 
upper  portion  of  the  stem 
and  the  immature  inflores- 
cence. The  axillary  buds 
of  the  upper  part  of  the  stem,  which  might  otherwise  have  produced 
secondary  branches  of  the  inflorescence,  and  which  would  then  have 


Yio.  6.— Part  of  the  stem  of  a  specimen  of  Sium 
cicutae/oliiim  with  three  rejuvenated  buds. 
Drawn  by  J.  Marlon  Shull  from  a  photograph. 


i6 


DEVELOPMENT    OF    SIUM    CICUTAEFOLIUM. 


borne  only  the  closing  members  of  the  senescent  series  of  leaf-forms 
(fig.  3),  were  rejuvenated  as  a  result  of  the  submergence  and  made  to 
begin  a  new  series,  the  early  members  of  which  show  the  most  interest- 
ing transitions  from  the  senescent  to  the  juvenile  forms.  Two  of  these 
rejuvenations  are  shown  in  figs.  5  and  6.     The  two  leaves  (n — 4,  n — 5) 


Fig.  6.— Two  rejuvenated  buds  from  the  upper  portion  of  a  8tem  of  tSiutn  cicutaefo- 
lium:  n— 6  and  n — I,  leaves  to  which  the  buds  were  axillary.  Numbers  represent 
the  position  of  each  leaf  In  the  bud.    Two  thirds  natural  size. 


at  the  left  in  fig.  6  are  the  leaves  to  which  the  two  rejuvenations  shown 
in  the  same  figure  were  axillary.  Unrejuvenated,  the  first  leaf  pro- 
duced by  each  of  these  buds  would  have  been  a  pinnate  leaf  of  exactly 
the  same  character  as  the  leaf  to  which  the  bud  was  axillary,  but  with  a 
less  number  of  leaflets.  Instead  of  this  we  see  in  each  case  a  leaf  hav- 
ing the  same  arrangement  of  parts,  to  be  sure,  but  greatly  modified  in 


PLATE  VI 


Sium  ciciUacJolium  Gmel.  The  first  four  leaves  of  the  sprins-srowth  of  an  old 
specimen.  Two-fifths  natural  size.  Photographed  April,  ic,Ov  from  a  specimen 
taken  into  the  house  February  25,  1905. 


DEVELOPMENT   OF    SIUM    CICUTAEFOLIUM. 


17 


the  direction  of  the  tripartite  leaf,  which  will  be  recalled  as  the  modal 
condition  in  the  first  nepionic  leaf.  In  the  second  leaf  in  the  one  case 
a  form  similarly  transitional  to  the  half-tripartite  leaf  occurred,  and  in 
the  other  case  the  juvenile  type  of  3-loJ|pd  leaf  was  perfected.  With 
the  third  leaf  in  the  one  rejuvenation  and  the  fourth  in  the  other,  a  pin- 
nate form  was  produced  which  corresponded  in  every  way  with  that 
seen  above  to  be  the  modal  condition  of  the  fifth  nepionic  leaf.  This 
was  as  far  as  one  of  these  rejuvenations  was  followed.  In  the  other 
the  fifth  leaf  had  five  pairs  of  leaflets,  a  degree  of  differentiation  not 


Fig.  7.— Leaves  of  a  rejuvenated  bud  from  near  the  base  of  a  stem  of  Sium  cicutae- 
/olium.  Letters  represent  In  a  general  way  the  position  of  each  leaf  In  the  reju- 
venation;   J/,  an  abnormally  divided  blade.     Four-ninths  natural  size. 


usually  attained  during-  the  first  year  from  seed.  Later  these  rejuvena- 
tions produced  the  dissected  leaves  characteristic  of  the  autumn  and 
early  spring  growth. 

Lateral  buds  at  the  base  of  the  flo\^(|ping  stems  are  regularly  rejuve- 
nated in  the  autumn,  and  the  first  leaves  are  pinnate  with  one  or  several 
pairs  of  undivided  leaflets  and  are  followed  by  a  linear  series  leading 
more  or  less  quickly  to  the  dissected  condition  (fig.  7) .  It  will  be  noted 
that  these  basal  rejuvenations  never  present  leaves  suggestive  of  the 
earliest  juvenile  stages,  as  is  the  case  in  the  rejuvenescence  of  buds 


i8 


DEVELOPMENT    OF    SIUM    CICUTAEFOLIUM. 


near  the  inflorescence,  probably  because  their  senescence  has  not  pro- 
gressed so  far. 

Senescence  reaches  its  culmination  in  the  region  of  the  flower.    Re- 
markably interesting  conditions  were  found  in  the  inflorescence  men- 


Fig  8.— Proliferations  (re- 
juvenations) of  an  Inflor- 
escence of  Siu77i  cicutaefo- 
Hum.  Drawn  by  J.Marlon 
Shull  from  a  photograph. 


tioned  above  which  had  been 
submerged  before  the  devel- 
opment of  the  flowers.  All 
the  flowers  had  given  place  to 
proliferations  (fig.  8),  i.  e., 
the  flower-buds  were  rejuve- 
nated by  the  submergence; 
and  just  as  the  flowers  repre- 
sent the  extreme  of  the  senes- 
cent condition,  the  prolifera- 
tions presented  extreme  juve- 
nile conditions,  showing  sev- 
eral types  of  leaf  which  are 
simpler  than  any  which  were 
found  in  the  seedlings,  and 
usually  reaching  the  condition  of  the  first  ncpionic  leaf  at  the  third  or 
fourth  leaf  of  the  proliferation.     Some  of  the  proliferations  were  more 


DEVELOPMENT    OF    SIUM    CICUTAEFOLIUM. 


19 


precocious  than  this,  however,  and  all  showed  a  much  more  rapid  devel- 
opment of  the  dissected  form  than  is  seen  in  the  seedling. 

The  simplest  leaf  presented  by  these  proliferations  (fig.  9)  was  a 
minute  oblong  undivided  structure  which  might  be  slightly  widened 
and  flattened  in  the  distal  half,  but  otherwise  showed  no  distinction  of 
blade  and  petiole.    This  might  be  followed  by  another  undivided  leaf 


Fig.  9.— Proliferations  froni  an    inflorescence  of  Sium  cicutaefolium.     Numbers 
show  position  of  each  leaf  in  its  own  proliferation.    Variously  magnlfled,  x  6  to  x  8. 

with  a  wider,  elliptic  blade.  Then  there  might  be  a  leaf  having  one  or 
a  pair  of  simple  lateral  lobes,  followed  perhaps  by  the  5-lobed  leaf  or 
the  form  which  has  been  described  as  the  simplest  type  of  3-lobed  leaf 
occurring  in  the  seedling,  having  a  2-notched  terminal  lobe  and  bifid 
lateral  lobes.  Whether  the  juvenile  stages  are  atavistic  or  not,  these 
early  leaves  of  the  proliferations  logically  precede  and  lead  up  to  them, 
and  carry  us  down  the  series  to  the  siinplest  possible  type  of  leaf. 


20  DEVELOPMENT    OF    SIUM    CICUTAEFOLIUM. 

The  rejuvenescence  of  all  these  buds  as  the  result  of  a  change  from 
an  aerial  to  an  aquatic  habitat  is  in  perfect  agreement  with  Burns's 
( 1904)  interpretation  of  the  changes  induced  in  a  similar  way  in  Proser- 
pinaca  pahistris  and  very  greatly  strengthens  his  position.  The  fact 
that  the  juvenile  leaves  of  that  species  are  finely  dissected  in  the  manner 
so  frequently  found  in  the  climactic  or  ephebolic  stages  of  typical  sub- 
merged plants  is  either  purely  a  coincidence  or  may  be  related  in  some 
unknown  way  to  the  past  environmental  relations  of  the  species,  as  sug- 
gested by  Goebel  (1899-1901,  p.  546).  In  Stum,  where  the  juvenile  leaf 
is  not  at  all  of  the  hydrophytic  type,  submergence  in  water  does  not 
produce  a  dissected,  hydrophytic  leaf,  but  the  mesophytic  form  of  leaf 
characteristic  of  the  Sium  seedling. 

This  change  of  view  regarding  the  effect  of  water  in  producing  the 
modified  leaf-form  of  these  plants  does  not  detract  in  the  least  from  the 
value  of  the  negative  results  of  McCallum's  (1902)  investigation  into 
the  nature  of  the  stimulus  involved,  but  gives  those  results  a  bearing  on 
the  phenomenon  of  rejuvenescence  instead  of  the  change  from  a  terres- 
trial to  an  aquatic  type  of  leaf.  To  ascribe  a  phenomenon  to  good  or 
poor  vegetative  conditions  (Burns,  1904,  p.  586)  does  not  yet  trace  it 
very  near  to  its  ultimate  cause  or  causes,  but  perhaps  is  as  definite  as 
the  present  state  of  our  knowledge  regarding  rejuvenescence  would 
warrant.  The  great  vigor  of  the  rejuvenated  buds  in  Sium  is  scarcely 
consistent  with  the  view  that  they  are  due  to  starvation  or  any  other 
poor  vegetative  conditions,  and,  indeed,  when  we  find  flower-buds,  rep- 
resenting as  they  do  the  low  ebb  of  vegetative  vigor,  suddenly  awakened 
into  a  new  cycle  of  vegetative  development  we  should  assume  that  they 
have  found  good  rather  than  poor  vegetative  conditions. 

It  is  evident  that  a  distinction  must  be  made  here  between  the  cause 
of  rejuvenescence  or  the  reawakening  into  the  ascending  side  of  the 
vegetative  cycle  and  the  cause  which  determines  just  what  point  in  the 
cycle  shall  be  attained  in  any  specific  case.  I  would  offer  the  tentative 
suggestions,  (a)  that  a  process  of  senescence  resulting  in  a  checking 
or  a  cessation  of  growth  is  a  necessary  condition  antecedent  to  reju- 
venescence, and  (b)  that  the  cause  of  the  reawakening  may  be  due  to 
either  or  both  of  two  complex  factors,  namely,  an  increase  in  available 
food-equivalent  and  an  increased  lability  or  mobility  (perhaps  largely 
fluidity)  of  the  protoplasmic  substances.  The  former  may  be  predomi- 
nantly o];)crativc  when  lateral  buds  are  forced  into  development  by 
checking  the  growth  of  a  terminal  bud,  and  the  latter  may  be  the  domi- 
nant factor  in  cases  of  submergence,  (c)  The  causes  which  determine 
the  point  in  the  cycle  which  shall  be  attained  under  any  specific  condi- 


DEVELOPMENT    OF    SIUM    CICUTAEFOLIUM.  21 

tion  may  well  be  the  relative  degree  of  one  or  other  of  these  factors  or 
of  their  resultant,  as  compared  with  that  necessary  for  the  produc- 
tion of  the  climax  type  of  leaf  of  the  adult  plant.  This  is  essen- 
tially in  accord  with  Burns's  (1904,  p.  586)  suggestion  of  poor  vegeta- 
tive conditions,  and  also  with  the  earlier  observations  of  Cushman 
(1902,  p.  885)  that  weak  individuals  and  those  growing  on  poor  soil  or 
with  insufficient  moisture  present  earlier  stages  than  do  more  vigorous 
specimens  or  those  growing  under  more  favorable  conditions  with  re- 
spect to  soil  and  water-supply.  As  applied  to  Proserpinaca  this  would 
mean  that  the  juvenile  type  of  leaf  in  the  submerged  plant  is  primarily 
the  result  of  protoplasmic  dilution,  but  that  the  failure  to  attain  the 
adult  condition  is  due  to  less  favorable  relations  as  to  food-supply.  Or- 
dinarily in  submerged  plants  of  Proserpinaca  these  two  factors  may  be 
conceived  to  balance  each  other  in  such  a  way  as  to  produce  the  dis- 
sected leaf  continuously,  but  an  unusually  vigorous  plant  may  be  able 
to  provide  a  food-supply  sufficient  to  overbalance  the  effect  of  dilution, 
and  so  give  rise  to  the  adult  type  of  leaf  which  is  occasionally  seen 
even  in  submerged  plants.  When  the  dissected  leaf  is  produced  in  aerial 
parts  it  is  primarily  due  to  a  condition  as  to  food  supply  better  than 
that  available  in  the  resting  condition,  but  not  equal  to  that  necessary 
for  the  production  of  the  adult  leaf-type.  With  the  return  of  spring, 
conditions  for  food-manufacture  are  improved  and  the  adult  leaf  ap- 
pears at  once. 

An  explanation  of  senescence  in  the  same  terms  would  ascribe  it  to 
decreasing  lability  or  fluidity  unaccompanied  by  a  compensatory  in- 
crease in  the  available  food-equivalent — a  condition  coming  just  as 
fully  under  the  head  of  poor  vegetative  conditions  as  does  that  pro- 
ducing the  juvenile  stages. 

Not  only  are  different  regions  of  the  axis  supposed  to  bear  leaves 
characteristic  of  dift'erent  periods  in  the  history  of  the  species,  but  it 
is  maintained  by  both  Jackson  (1899)  and  Cushman  (1902,  1903)  that 
certain  parts  of  the  leaf,  especially  the  apical  region,  likewise  show 
atavistic  conditions.  Leaving  wholly  aside  for  the  present  the  relation 
between  the  parts  of  the  present  adult  leaf  and  the  form  of  the  ancestral 
leaf,  the  question  involved  in  this  proposition  is  the  localization  of  the 
differentiative  activities  in  the  leaf.  The  simpler  character  of  the  apical 
region  in  many  leaves  is  undeniable,  and  it  may  correspond  in  certain 
cases,  therefore,  with  ancestral  conditions,  though  this  has  not  been 
proved  in  any  specific  case.  But  the  simplicity  of  the  apical  region  has 
led  both  of  these  writers  into  error  when  they  conclude  that  new  char- 
acters arise  at  the  base  of  the  leaf.    After  examining:  manv  leaves  and 


22 


DEVELOPMENT    OF    SIUM    CICUTAEFOLIUM. 


also  the  figures  upon  which  Jackson  and  Cushman  base  their  conclu- 
sions, I  am  convinced  that  with  possible  rare  exceptions  the  new  char- 
acter added  to  a  leaf  having  a  single  blade  and  an  entire  margin  consists 
in  an  indentation  or  an  incision  rather  than  an  outgrowth,  and  that,  in 
general,  increased  complexity  is  brought  about  by  an  increasing  number 
and  depth  of  these  incisions,  which  do  not  and  can  not  occur  at  the 
base,  but  must  occur  above  it.  That  part  of  the  base  below  the  lowest 
incision  is  characterized  by  an  entire  margin  and  is  "  primitive,"  there- 
fore, in  the  same  sense  as  is  that  part  of  the  apex  above  the  highest 
incision.  Just  as  Cushman  (1903,  p.  244)  noted  that  emarginate  apexes 
may  develop  from  acute  ones  in  Astragalus  adsurgens,  so  in  cases  like 
that  of  his  Arabis  albida,  in  which  truncate  and  cordate  bases  occur  in  a 
series  beginning  with  attenuate  bases,  these  forms  are  late  modifications 
of  a  tapering  base,  due  to  the  excess  of  marginal  growth  as  compared 
with  that  of  the  mid-rib.  Although  the  basal  margin  is  thus  distorted 
from  its  original  direction,  it  retains  its  unbroken  "primitive"  character. 

The  figures  which 
illustrate  Cushman's 
papers  show  these 
facts  so  clearly  that 
it  seems  strange  that 
he  should  not  have 
recognized  them. 
Particularly  in  Sib- 
bald'wpsis{PotentiUa) 
tridenata,  Artemisia 
stelleriana,  and  San- 
gnisorba  (Poteriuui) 
canadensis  is  it  diffi- 
cult to  harmonize  his 
figures  with  his  state- 
ment that  new  charac- 
ters are  produced  at 
base.  In  the  first  two 
of  these  plants  the  new 
character,  as  I  under- 
stand it,  consists  in  the  appearance  of  two  small  indentations  very  near 
the  apex.  In  Sanguisorba  canadensis  the  leaf  is  pinnate,  and  in  the 
later  stages  of  the  senescent  series  each  proximal  leaflet  presents  in  each 
leaf  a  peculiar  inequilateral  form,  though  the  number  of  leaflets  de- 


FiG.  10.— Diagram  to  test  by  the  law  of 
periodicity  tlae  serial  homology  of 
leaflets  in  a  pinnate  leaf.  Horizontal 
lines  represent  the  rachises  of  the 
successive  leaves  of  a  stem  of  Siuin 
cicutae folium .  Nodes  assumed  to  be 
homologous  are  connected  by  curves. 
The  proximal  leaflets  assumed  to  be 
homologous. 


DEVELOPMENT    OF    SIUM    CICUTAEFOLIUM. 


23 


creases  regularly  as  the  inflorescence  is  approached.  In  order  to  make 
this  leaf  fit  his  view  of  basal  differentiation,  Cushman  (1903,  p.  254) 
assumes  that  as  each  proximal  leaflet  disappears  the  next  higher  leaflet 
takes  on  this  same  inequilateral  form. 

According  to  this  view  the  proximal  leaflets  are  the  newest.  If,  on 
the  other  hand,  it  be  conceived  that  the  leaflets  appear  in  apical  succes- 
sion by  the  division  of  the  terminal  leaflet,  and  that  they  disappear 
through  the  loss  of  one  pair  of  incisions  after  another,  proceeding  now 
basipetally,  the  proximal  leaflets,  which  have  in  every  late  senescent 
leaf  the  same  peculiar  shape,  will  be  the  oldest  instead  of  the  newest, 
and  the  inequilateral  leaflets  of  one  leaf  will  be  homologous  with  those 
of  all  of  the  other  leaves.  In  the  latter  case  it  would  occasion  no  sur- 
prise to  find  that  these  leaflets  show  a  well-established  form  which 
occurs  with  considerable  constancy  in  each  succeeding  leaf.  If  these 
proximal  leaflets  were  in  a  state  of  perpetual  nascence  and  evanescence 
we  would  expect  the  process  to  result  in  frequent  imperfect  or  incom- 
plete differentiations 
and  consequent  great 
variation,  a  condition 
never  realized. 

Every  one  will  re- 
call frequent  instances 
in  which  the  terminal 
leaflet  of  a  pinnate 
leaf  showed  imperfect 
formation  of  lateral 
leaflets  by  incisions 
of  greater  or  less 
depth  near  its  base 
(n— 4  and  y,  fig.  3), 
and  every  degree  of 
division  will  have 
been  noted,  from  a 
slight  notch  to  the 
complete  formation  of 
a  pair  of  adjacent 
leaflets.  This  ap- 
pears to  me  to  be  the  best  possible  evidence  that  in  this  region,  just 
above  the  base  of  the  terminal  leaflet,  is  the  place  of  progressive  differ- 
entiation and  reintegration  in  pinnate  leaves,  and  that  it  is  here  and  not 
in  the  proximal  leaflets  that  new  characters  are  to  be  looked  for. 


Fig.  U.— The  same  as  flg.  10,  but 
the  distal  leaflets  are  assumed 
to  be  homologous. 


24  DEVELOPMENT    OF    SIUM    CICUTAEFOLIUM. 

As  further  evidence  that  this  is  the  correct  interpretation  of  the  pin- 
nate leaf,  the  diagrams  shown  in  figs.  lo  and  ii  were  constructed  as 
follows  :  All  the  leaves  of  a  single  stem  of  Sium  cicutaefolhim  (omitting 
several  which  had  disappeared  at  the  base)  were  arranged  in  their 
normal  sequence,  but  side  by  side,  with  their  rachises  parallel  and  at 
equal  intervals.  These  rachises  are  represented  by  the  horizontal  lines 
of  the  figures.  The  point  of  origin  of  each  pair  of  leaflets  was  then 
marked,  and  the  figures  were  completed  by  connecting  with  lines  the 
points  of  origin  of  leaflets  assumed  to  be  homologous.  In  fig.  lo  the 
proximal  leaflets,  and  in  fig.  ii  the  distal  leaflets,  are  assumed  to  be 
homologous.  The  basis  for  the  interpretation  of  these  figures  is  to  be 
found  in  the  demonstration  by  Fraulein  Tammes  (1903)  that  the  law 
of  periodicity  underlies  all  morphological  phenomena  in  such  a  way 
that  serially  homologous  characters  increase  regularly  to  a  maximum 
and  then  decrease,  decrease  regularly  to  a  minimum  and  then  increase, 
or  that  they  form  a  half  period,  i.  e.,  they  begin  with  the  maximum  and 
end  with  the  minimum,  or  vice  versa.  Even  a  most  superficial  inspec- 
tion of  the  two  figures  discloses  the  fact  that  in  one  there  is  a  simple 
underlying  law  which  would  lend  itself  to  ready  formulation,  while  in 
the  other  all  is  confusion.  The  characters  which  are  represented  in 
these  figures  are  the  interfoliola  or  portions  of  the  rachis  between  suc- 
cessive pairs  of  leaflets.  Homologous  interfoliola  are  represented  by 
the  portions  of  the  horizontal  lines  included  between  any  two  consecu- 
tive curves.  In  fig.  10  these  interfoliola  are  seen  to  reach  a  definite 
maximum  length  in  the  second  leaf  from  the  base  of  the  figure  and  to 
diminish  continuously  from  that  maximum  upward  until  each  in  turn 
is  reduced  to  zero,  as  required  by  the  law  of  periodicity.  The  only 
irregularities  appear  in  the  two  distal  interfoliola  of  the  third  leaf  from 
the  base  of  the  figure  and  in  the  distal  interfoliolum  of  the  first  and  of 
the  fifth  leaves  from  the  base,  these  variations  in  the  distal  interfoliola 
being  in  full  accord  with  my  view  that  this  is  the  region  of  active  dififer- 
entiation  and  reintegration  in  the  leaf.  In  fig.  11,  on  the  other  hand, 
there  is  not  a  single  instance  in  which  a  series  of  interfoliola  assumed 
to  be  homologous  shows  an  increase  to  a  definite  maximum  followed  by 
a  continuous  decrease.  The  law  of  periodicity  is  beautifully  exemplified 
in  fig.  10,  which  is  based  on  the  assumption  that  the  proximal  leaflets 
of  one  leaf  are  homologous  with  the  proximal  leaflets  of  every  other  leaf 
of  the  same  stem,  while  there  is  no  indication  of  that  law  in  fig.  ii,  in 
which  the  distal  leaflets  are  assumed  to  be  homologous.  Certainly  no 
more  conclusive  proof  of  the  truth  of  my  proposition  could  be  asked. 

As  in  the  case  of  the  juvenile  and  senescent  leaves,  the  sole  basis  for 
the  assumption  that  the  base  and  apex  of  a  leaf  exhibit  conditions  which 


DEVELOPMENT    OF    SIUM    CICUTAEFOLIUM.  25 

were  characteristic  of  the  whole  adult  leaf  at  some  time  in  the  past 
history  of  the  species  is  the  fact  of  their  greater  simplicity.  The  rela- 
tion is  purely  a  logical  one,  and  the  condition  of  these  regions  of  the  leaf 
are  not  unlikely  to  disagree  with  phylogenetic  conditions  in  every  detail 
except  simplicity. 

Although  this  study  makes  it  evident  that  no  satisfactory  inferences 
can  be  drawn  from  the  forms  occurring  at  various  ontogenetic  stages 
regarding  the  ancestral  adult  conditions,  these  forms  do  have  a  bearing 
upon  the  relationship  of  allied  species.  The  similarity  of  two  species 
as  to  the  characters  possessed  at  any  of  these  stages  would  lend  evidence 
in  favor  of  their  close  relationship  in  the  same  way  that  similarity  in 
any  other  activity  or  character  would,  not  because  of  resemblance  to  a 
common  ancestral  adult  condition,  but  because  of  the  presumptive  evi- 
dence of  similarity  in  present  protoplasmic  structure. 

All  the  evidence  now  available  indicates  that  when  specific  differen- 
tiation takes  place  the  changed  structure  of  the  protoplasm  which  pro- 
duces new  adult  characters  also  gives  rise  to  new  characters  in  the 
juvenile  and  senescent  series.  The  assumption  that  these  stages  have 
a  phylogenetic  significance  tends  to  obscure  the  fact  that  they  are  the 
results  of  present  conditions  instead  of  the  past  history  of  the  proto- 
plasm and  that  they  are  in  need  of  physiological  rather  than  phylo- 
genetic investigation  and  interpretation. 


2.(i  DEVELOPMENT   OF    SIUM    CICUTAEFOLIUM. 

SUMMARY. 

The  various  forms  of  leaf  which  occur  at  different  parts  of  a  speci- 
men of  Slum  ciciitaefoliiim  are  described  and  their  bearing  on  the  phy- 
logenetic  history  of  the  species  is  considered.  The  principal  conclu- 
sions are  as  follows : 

There  is  great  variability  in  the  early  nepionic  leaves,  the  first  leaf 
being  the  most  variable  and  the  variability  being  progressively  lessened 
in  subsequent  growth. 

The  interpolation  of  an  undivided  leaf  between  the  3-lobed  and  the 
pinnate  leaf  is  illogical  and  might  be  interpreted  as  representing  phylo- 
genetic  mutation. 

To  show  how  wholly  gratuitous  is  the  attempt  to  draw  conclusions 
from  juvenile  stages  regarding  ancestral  adult  conditions,  it  is  pointed 
out  that  in  the  species  of  Onagra,  where  the  ancestry  is  definitely 
known,  the  juvenile  leaves  may  resemble  more  closely  the  adult  leaves 
of  the  same  species  than  they  do  either  the  seedling  or  the  adult  leaves 
of  the  parent  species. 

The  senescent  stages  are  totally  different  from  those  of  the  seedling. 
The  leaves  show  a  regular  reduction  without  sudden  changes  of  type, 
and  there  is  increasing  constancy  in  form  as  higher  stages  are  reached. 

Although  the  several  stages  of  Sium  present  the  appearance  of  being 
closely  correlated  with  the  successive  changes  in  its  environment,  the 
plant  passes  through  the  same  stages  whenever  the  conditions  are  favor- 
able for  growth,  regardless  of  the  characteristics  in  the  environment 
which  have  been  supposed  to  determine  the  several  types  of  structure. 

This  cycle  of  development  can  be  repeated  only  through  rejuve- 
nescence. This  may  result  from  fertilization  and  seed-production, 
but  can  be  brought  about  in  other  ways.  Rejuvenations  of  axillary 
buds  and  of  flower-buds  are  figured  and  described.  Rejuvenescence 
in  these  buds  was  caused  by  submergence  in  water  of  stems  in  the 
senescent  state.  The  later  the  stage  of  senescence  reached  the  earlier 
are  the  juvenile  stages  produced  on  rejuvenescence.  Proliferations  of 
flower-buds  showed  several  leaf-forms  which  are  simpler  than  any 
found  in  the  seedlings. 

Rejuvenescence  may  be  due  to  increased  available  food-equivalent, 
or  to  increased  lability  or  fluidity  of  the  protoplasmic  substances,  or  a 
combination  of  these  two  factors.  The  stage  attained  in  any  case  of 
rejuvenescence  may  be  due  to  the  relative  value  of  the  resultant  of  these 
two  complex  factors,  as  compared  with  that  necessary  to  the  production 
of  the  climax  type  of  leaf  of  the  adult  j)lant. 


DEVELOPMENT    OF    SIUM    CICUTAEFOLIUM.  2^ 

It  has  been  claimed  that  the  apex  of  a  leaf  presents  primitive  condi- 
tions and  that  new  leaf-characters  appear  at  the  base  of  the  blade.  It 
is  shown  here  that  the  base  is  also  "  primitive  "  and  that  the  new  char- 
acters appear  above  the  base. 

The  proximal  leaflets  of  pinnate  leaves  on  the  same  stem  are  homolo- 
gous, as  are  all  other  pairs  of  leaflets  having  positions  of  like  order, 
counting  from  the  proximal  pair. 

The  sole  basis  for  the  assumption  that  localized  stages  present  ata- 
vistic characters  is  the  fact  of  their  greater  simplicity.  No  satisfactory 
inferences  can  be  drawn  from  ontogenetic  leaf-characters  regarding 
the  phylogenetic  history  of  the  species.  The  various  stages  are  the 
result  of  present  protoplasmic  structure  instead  of  the  past  history  of 
the  protoplasm,  and  a  change  of  structure  which  results  in  new  adult 
characters  may  also  produce  changed  juvenile  and  senescent  characters. 
They  are  in  need  of  physiological  instead  of  phylogenetic  interpretation. 

Station  for  Experimental  Evolution, 

Cold  Spring  Harbor,  N.  Y.,  December,  1904. 


28  DEVELOPMENT    OF    SIUM    CICUTAEFOLIUM. 

LITERATURE  CITED. 

Agassiz,  Louis. 

1848-1849.     Twelve   lectures   on   comparative    embryology.     Boston:    Lowell 

Institute. 

Braun,  Alexander. 

1851.  Reflections  on  the  phenomenon  of  rejuvenescence  in  nature,  especially 
in  the  life  and  development  of  plants.  Translation  from  the  German,  by 
A.  Henfrey.    London:  Ray  Society,  1853. 

Burns,  George  P. 

1904.     Heterophylly  in  Proserpinaca  palustris  L.   Annals  of  Botany,  18:  579- 
587. 
CusHMAN,  Joseph  A. 

1902.  Studies  of  localized  stages  of  growth  in  some  common  New  England 
plants.     Amer.  Nat.,  36:  865-885. 

1903.  Studies  of  localized  stages  in  some  plants  of  the  botanic  gardens  of 
Harvard  University.     Amer.  Nat.,  37:  243-259. 

1904.  Localized  stages  in  common  roadside  plants.     Amer.  Nat,  38:  819-832. 

Glaser,  Otto  C. 

1902.  The  law  of  von  Baer.     Science,  N.  S.,  1 5  :  976-982. 

Goebel,  Karl. 

1898-1901.  Organographie  der  Pflanzen  insbesondere  der  Archegoniaten  und 
Samenpflanzen.     pp.  xviii,  838.     Figs.  539.     Jena :  Gustav  Fischer. 

Haeckel,  Ernst. 

1866.     Generelle  Morphologic  der  Organismen,  2  Bde.     Berlin :  G.  Reimer. 

Hyatt,  Alpheus. 

1890.     Genesis  of  the  Arietidae.     Smithsonian  Contribution  673,  and  Mem. 

Mus.  Comp.  Zool.,   16: 
1897.    Cycle  in  the  life  of  the  individual  (ontogeny)  and  in  the  evolution  of 

its  own  goup    (phylogeny).    Proc.  Amer.  Acad.  Arts  and  Sciences,  32: 

209-224. 

Jackson,  R.  T. 

1899.  Localized  stages  in  development  in  plants  and  animals.  Proc.  Boston 
Soc.  Nat.  Hist.,  5  :  89-153. 

Klebs,  G. 

1903.  Willkiirliche  EntwicWungsanderungen  bei  Pflanzen.  pp.  iv,  166.  Figs. 
28.    Jena :  Gustav  Fischer. 

McCallum,  W.  B. 

1902.  On  the  nature  of  the  stimulus  causing  the  change  of  form  and  structure 
in  Proserpinaca  palustris.     Bot.  Gaz.,  34  :  93-108. 

TammES,  Fraulein  TinB. 

1903.  Die  Periodioitat  imorphologischer  Ersdheinungen  bei  den  Pflanzen. 
Verhandelingen  d.  Koninklijke  Akad.  v.  Wetenschappen  te  Amsterdam, 
9  (No.  5)  :  1-148. 

Von  Baer,  K.  E. 

1828.  Ueber  Entwicklungsgesdhiohtc  der  Thiere.  Beobadhtungen  und  Re- 
flexion.   Konigsberg:  Gobr.  Borntrager. 


i^' 


UNIVERSITY  OF  CALIFORNIA  LIBRARY 

Los  Angeles 
This  book  is  DUE  on  the  last  date  stamped  below. 


UNIVERSITY  of  CALIFORNIA 

AT 

LOS  ANGELES 

LIBRARY 


UCSOlJTHfH',  K(i.!'/,Ai  i,  :RRARVr;.- 


AA    000  876  421     9 


''':':^''yf<;m 


I'  'A 


